Conventional magnetic recording elements that are used for recording sounds or images are generally opaque to visible light regardless of the nature of the magnetic particles used in such elements. For example, motion picture films often are provided with a magnetic sound track which generally is opaque and does not cover that portion of the film used in the projection of images.
Canadian Patent 686,172 shows that a magnetic recording layer may be transparent to visible light when it contains low concentrations of magnetizable particles. According to this patent, such a layer is coated over a layer containing descriptive material which allows a user to simultaneously hear and see certain subject matter. However, this patent points out that the electromagnetic characteristics, i.e., the magnetic recording and reproducing characteristics, of such a layer are inferior to those of conventional magnetic layers as a result of the very low concentration of magnetizable particles.
U.S. Pat. No. 3,782,947 discloses a photographic product which carries magnetic particles distributed across the image area of the product. The particle distribution and sizes are so designed that the composite granularities of the photographic and magnetic recording media are such that the magnetic distribution is essentially transparent in a photographic sense. According to this patent, the photographic image can be viewed via the magnetic distribution and the magnetic distribution can be employed for recording and playing back information.
U.S. Pat. No. 4,279,945 discloses a process of preparing magnetic recording elements containing a transparent recording layer. According to this patent, the magnetic recording and reproducing characteristics obtained are comparable to conventional opaque magnetic layers without the need for matching the granularity of a magnetic medium to that of photographic medium. However, the process requires that the layer containing magnetic particles be treated using one or both of the following process steps, (1) compacting the layer while it is in a malleable state to reduce its thickness (e.g., calendaring), or (2) imbibing into the layer a substantially transparent liquid having a refractive index that is substantially the same as that of the binder.
Elements of the type described in the above-cited patent have not achieved widespread commercial success for various reasons. For example, the elements described in U.S. Pat. No. 4,279,945, as indicated by the Figure therein, are substantially opaque at wavelengths less than about 500 nm and thus are not useful in color films. Further, the disclosed process requires that the magnetic recording layer be calendared while it is in a malleable state and/or that a transparent liquid be imbibed into the magnetic recording layer. On the other hand, U.S. Pat. No. 3,782,947 contemplates coating a dispersion containing magnetic particles onto a film base. However, the quantity of solvent required in such a process is unattractive from both an economic and environmental standpoint. Additionally, in continuous wide web coating techniques adapted for commercial manufacturing operations, solvent attack on the film base and can render the element unusable, resulting in unacceptable manufacturing inefficiencies and excessive costs. Moreover, it is difficult to prepare magnetic recording layers in such a coating process having a thickness of less than about 5 microns.
U.S. Pat. No. 4,990,276 discloses a dispersion of magnetic particles, a dialkylester of phthalic acid and a dispersing agent. U.S. Pat. No. 5,254,449 discloses the use of such a dispersion to provide a substantially transparent magnetic recording layer in the preparation of a novel photographic element. The process described in this patent calls for dilution of the dispersion with a binder in an organic solvent such as a mixture of methylene chloride and methanol, to form a casting composition. The casting composition is then extruded under pressure onto a photographic support.
It is evident that it would be desirable to provide a photographic element with a magnetic layer of the desired transparency without the use of an organic solvent. Further, it is evident that it would be desirable to provide a stable aqueous dispersion of magnetic particles from which to prepare such an element.